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The results of our equilibrium and calorimetric studies show (Table I) that the greater stability of the Table I.
Stability Constants, Enthalpy, and Entropy of Formation
of the Nickel Complexes of 2,3,2-tet and cyclam at 25.0" and p = 0.1 Complex Ni(2,3,2-tet)2+ [Ni(2,3,2-tet) (sq pIanar)]z+ Ni(cyclam)2+
log KNiL 15.8 15.4
-19.4 i 0.lbah -16.8 i 0 . 5
22.2 =!=
AH", kcal/mol
AS", cal/ ("K mol) 7.4 14
-31.0 i 0 . 6 L , h * s - 2 0.2d+
Log K N ~isL16.4 in 0.5 MKC1 (see footnotef) and is corrected to 0.1 (see footnotesfand g ) . * Determined by calorimetry using the reaction with CN- and A H " for Ni(CN)42-. Log K, A H " , and AS" are corrected for the blue e yellow equilibrium (ref 7). Determined spectrophotometrically using CN- and correcting for the Ni(cyclam)CN+ and Ni(cyclam)OH+ c o m p l e x e ~ . ~e Determined from the temperature dependence (10.0-40.0") of the equilibrium constants.9 f D. C. Weatherburn, E. J. Billo, J. P. Jones, atid D. W. Margerum, Inorg. Chem., 9, 1557 (1970). 0 L. Sacconi, P. J. J. Paoletti, and M. Ciampolini, J. Chem. SOC.,5115 (1961). Christensen, R. M. Izatt, J. D . Hale, R. T. Pack, and G. D. Watt, Inorg. Chem., 2, 337 (1963). The AH' value for Ni(CN)2- is interpolated to p = 0.1 from the tabulated values a t ionic strengths of 0.082 and 0.134. G. B. Kolski and D. W. Margerum, Znorg. Chem., 7,2239 (1968). 4
p =
macrocyclic complex actually is due to a more favorable change in AH" (a difference of -14 kcal/mol) which overcomes a less favorable change in AS" (a difference of -16 cal/("K mol)) for the reaction. Up to this point the effect of ligand solvation has been neglected. In fact, ligand solvation has generally been ignored in the consideration of the thermodynamics of metal complexation reactions. However, for the reaction in eq 1 where the primary solvation ( y ) of the ligand L is Ni(H20),*+
+ L(H20), ----f NiL(H20),*' + ( x + y - z)H20
(1)
variable, the AH" value for the reaction will increase (be less negative) as the enthalpy of solvation of L increases and the AS" value will increase as y increases due to the additional HzO molecules which are released in the reaction. We propose that AH" is much more negative for the complexation of cyclam because this ligand is less solvated by water than is 2,3,2-tet. Cyclam is not able to accommodate as many hydrogen-bonded water molecules with its nitrogen donor atoms because of steric hindrance. However, the primary solvation should be the same for both Ni(cyclam)2+ and Ni(2,3,2tet)*f. The result is that for cyclam less enthalpic energy need be expended in breaking hydrogen bonds with the solvent (AH" 'v 7 kcal/mol)8 and a more favorable AH" change is found for reaction 1. It follows that A S " must be less positive for the cyclam reaction than for the 2,3,2-tet reaction because fewer water molecules are released from the ligand. A difference of two in the number of ligand-solvating water molecules ( y ) accounts for the difference in AH" values of the cyclam and 2,3,2-tet reactions. The (8) H. Fritzshe, Ber. Bunsenges. Phys. Chem., 68, 459 (1964); E. M. Arnett, L. Joris, E. Mitchell, T. S. S. R. Murty, T. M. Gorrie, and P.
v. R. Schleyer, J . Amer. Chem. Sac., 92, 2365 (1970); R. A. Hudson, R. M. Scott, and S . N. Vinogradov, Spectrochim. Acta, Part A , 26, 337 (1970); A. D. H. Clague, G. Govil, and H. S . Bernstein, Can. J . Chem., 47, 625 (1969); P. A. Kollman and L. C. Allen, J . Amer. Chem. Soc., 93,4991 (1971). Average values of AH" and ASo were used.
Journal of tfie American Chemical Society
96:15
difference in the A S o values for the two reactions (16 cal/( OK mol)) is less than would be predicted for the release of two water molecules. This can be attributed to the configurational entropy difference of the ligands and the AS" value of + I 4 cal/("K mol) for the 2,3,2-tet reaction would be even larger compared to cyclam were it not for the greater loss of configurational entropy of the open-chain ligand. The postulated ligand solvation effect permits several predictions. The macrocyclic effect should be independent of the metal as long as there is not an unfavorable geometry in the coordination of the metal ion within the macrocycle. Thus, very similar enhancements of the stability constants are found for Cu2+and Ni2+with hexamethyl derivatives of ~ y c l a m . If ~ the solvent is changed to one with weaker primary solvation of the ligand then the importance of the solvation effect will diminish and the relative contribution due to the configurational entropy will increase. Preliminary results support this prediction. lo The predicted ligand solvation effects are not restricted to macrocyclic ligands but will hold for any ligands where the donor groups are forced to be close to one another or in some way are shielded from solvation. In this respect the terminology of multiple juxtapositional fixedness, although awkward, is appropriate. Thus, the effect of ligand solvation on metal stability constants should be particularly important in biological systems. Obviously, the magnitude of metal ion binding constants to proteins and other biological species may be very much influenced by solvation and could differ from that of smaller, more flexible molecules by very large factors. Finally, there are many small variations in the stability constants of coordination complexes which have been explained by a variety of other postulated effects. These now need to be reexamined in view of the possible magnitude of ligand solvation effects. Acknowledgment. This work was supported by National Science Foundation Grant G P 14781X. (9) F. P. Hinz and D. W. Margerum, Inorg. Chem., in press. (10) G. F. Smith and D. W. Margerum, to be submitted for publication. Frederick P. Hinz, Dale W. Margerum* Department of Chemistry, Purdue Unioersiiy West Lafayette, Indiana 47907 Received April 9, 1974
Rhodium(1) Dithiolene Complexes. Structure, and Dynamic Behavior
Synthesis,
Sir: Studies of Rh(1) complexes have been stimulated by their role in homogeneous catalytic reactions and the related oxidative addition chemistry. T o date, the overwhelming majority of these systems are neutral or cationic' with few exceptions such as Rh(CO)&- 2a (1) See, for example, (a) F. A. Cotton and G. Wilkinson, "Advanced Inorganic Chemistry," 3rd ed, Interscience, New York, N. Y.,1973, p 1018; (b) C. K. Brown and G. Wilkinson, J. Chem. SOC.A, 2753 (1970), and references therein; (c) M. Green, T. A. Kuc, and S. H. Taylor, ibid., 2334 (1971); (d) R. R.Schrock and J. A. Osborn, J . Amer. Chem. Soc., 93, 3089 (1971); (e) L. M. Haines, Inorg. Nucl. Chem. Lett., 5,399 (1969); Inorg. Chem., 9, 1517 (1970). (2) (a) L. M. Vallarino, Inorg. Chem., 4, 161 (1965); (b) N. G . Connelly and J. A. McCleverty, J . Chem. SOC.A, 1621 (1970).
1 July 24, 1974
4995
Figure 2. A perspective view of the structure of Rh(COD)(CHamnt). The estimated standard deviations in the bond lengths are: Rh-S, 0.002; S-C, 0.005; Rh-C, 0.005; C-C, 0.007 A. Some important bond angles with esd's in parentheses are: Sl-Rh-S2, 87.48 (7); Rh-SI-C13, 114.7 (2); Rh-S1-Cl, 105.0 (2); Cl-SI-C13 102.6 (2) ; Rh-S2-C2,103.2 (2) '.
point precipitation of the complex causes a loss in resolution. Similar dynamic behavior is also noted with Rh( COD) (mnt)( C6H&H2) and with Rh( NBDXmnt) (C6H5CH2). Reactions of the Rh(1) anions with SOs and CeH5HgC1are also observed. The SO, adducts are reversibly formed. The reaction involving C6H5HgCI results in a complex whose stoichiometry most closely resembles the neutral phenylmercury adduct; however, insolubility and lack of volatility have hindered more extensive characterization. For the purpose of explaining the variable temperaFigure 1. Temperature dependence of the 100-MHz lH nmr specture nmr spectra of the alkyl adducts, the structure of trum of the methylated complex "Rh(COD)(mnt)(CHa)" in C D Z C ~ Z . Rh(COD)(mnt)(CH3) was determined by X-ray methods: (crystal data) a = 14.78 (l), b = 10.69 (l), c = 9.20 (1) A; /3 = 106.09 (5)'; I/ = 1397 A; space and Rh(CO),(S-S)- where S-S is a bidentate sulfur group, P21/a - Gh5;Pexptl = 1.75 (2), Pcaied = 1.75 donor ligand. 2b We now report two new Rh(1) anionic g/cm3 for Z = 4; p(Mo Ka) = 14.67 cm-'. Intensity complexes, the reactions of these complexes with alkyl data were collected by the 8-28 scan technique on a halide substrates, and the structures and fluxional becomputer-controlled diffractometer using Mo Kcu havior of the novel adducts. The observed reaction radiation. The structure was solved by standard heavy chemistry implies that the anionic systems have two atom methods, and refined by least squares to discrepsites-the Rh(1) ion and the thiolate S donor-for ancy indices R and R ' of 0.032 and 0.045, respectively, potential attack on common oxidative addition subfor 2127 reflections above 2u. In the refinements all strates. nonhydrogen atoms were allowed to vibrate according The complexes Rh(diene) (mnt)- (diene = 1,5-cycloto an anisotropic thermal model. Hydrogen atoms octadiene (COD), and norbornadiene (NBD); mnt = were located and included in the final cycles as fixed maleonitriledithiolate) are prepared from the correcontributions with isotropic temperature factors. Final sponding chloro bridged dimers [Rh(diene) (p-Cl)lZ3by positional and thermal parameters for the structure are stoichiometric reaction with Naz(mnt) in ethanolic available. hydrazine. Addition of tetra-n-butylammonium broThe structure determination reveals that the complex mide, followed by solvent evaporation and recrystallizais methylated at one of the sulfur donor atoms rather tion from ethanol-hexane results in yellow crystalline than at the rhodium. The methylated complex thus samples of the Rh(1) anions as their (n-C4H&N+ salts. remains a four-coordinate d8 system, and a perspective The complexes react with CHJ and C6HjCH2X(X = view of the molecule is shown in Figure 2. Despite the Br, I) in ethanol to yield neutral adducts with analyses alkylation of one of the sulfur donor atoms, the two corresponding to the general formula Rh(diene)(mnt)R. Rh-S distances are nearly equal with the Rh-S (CH3) The lH nmr spectrum of Rh(COD)(mnt)(CHs) at amdistance being slightly shorter (see Figure 2). The two bient room temperature exhibits three resonances asS-C distances of the mnt ligand, however, are signifsigned to a methylene multiplet at 6 2.3, a methyl singlet icantly different reflecting the partial multiple bond at 6 2.68 and a broad vinyl resonance at 6 4.75. Upon character in the carbon-thiolate sulfur bond. The cooling, the vinyl resonance broadens, collapses, and COD ligand adopts a conformation such that the metal separates into two distinct multiplets at 6 4.99 and 4.53, diolefin chelate unit possesses only approximate C2 while the methyl resonance changes from a singlet to a symmetry with torsional angles about the bonds of the closely spaced doublet (JR~-H = 1.5 Hz). Figure 1 methylene C atoms averaging 93.7 (9), 33.4 (lo), and shows the temperature dependence of the 1H nmr spec41.8 (lo)". The methylated sulfur is pyramidal with trum of Rh(COD)(mnt)(Ch3) down to -20"; the the methyl group 1.42 A out of the coordination plane. spectrum remains invariant from -20 to -80" at which (3) (a) J. Chatt and L. M. Venanzi, J. Chem. Soc., 4735 (1957); (b) E. W . Abel, M. A . Bennett, and G. Wilkinson, ibid., 3178 (1959).
(4) See paragraph at end of paper regarding supplementary ma-
terial.
Comniunications to the Editor
4996
The four vinyl protons of the COD ligand are thus nonequivalent in the static structure (two cis to the methylated sulfur and two trans, two above the coordination plane and two below). Since only two vinyl resonances are observed in the low temperature spectrum down to -SO", we suggest that two dynamic processes are occurring in solution which equilibrate pairs of vinyl protons with only one of these processes observed on the nmr time scale in the present study. Possible averaging processes include : (a) inversion at the pyramidal s u l f ~ r ;(b) ~ twofold twist of one chelate ring going through a tetrahedral transition state; (c) dissociation of a Rh-olefin bond, rotation of the diolefin, and recombination; and (d) a process similar to (c) in which the Rh-S(CH3) bond cleaves. Because of the loss of methyl proton-rhodium coupling in going from -20" to room temperature, we tend to favor (d) as the observed process with inversion at sulfur (a) as the rapid equilibrating process which we are unable to freeze out on the nmr time scale. However, (b) cannot be ruled out as the observed process because of coalescence temperatures of 0 and - 50" for Rh(C0D) (C6H6CH2-mnt) and Rh(NBD)(CsH5CHz-mnt), respectively. In the twofold twist mechanism, steric interactions between the diene and the sulfur donor ligands should be reduced for the NBD complex and thus lead to the observed lower barrier for equilibration in that system. The low activation energy implicit in our assumption about inversion at sulfur seems reasonable since the transition state for process (a) should be stabilized by a delocalized dithiolene resonance structure. In addition, the solid state structure of Rh(C0D) (CHa-mnt) shows the alkylated sulfur to be slightly less pyramidal than those found in other metal complexes.6 The magnitude of in the present case is similar to that found in structurally related systems.' Further studies including detailed spectral analyses will be reported. Alkylation of dithiolate sulfur atoms has been observed previously.* In the present study the adduct complexes remain as d8 Rh(1) systems potentially capable of undergoing oxidative addition reactions. The thiolate S donor lone pairs function as nucleophiles in attack on oxidative addition substrates and thus raise the possibility of utilizing such ligand lone pairs in certain multi-step, metal complex promoted reactions. Acknowledgment. We thank the National Science Foundation (GP-40088X) for support of this research (5) (a) P. Haake and P. C. Turley, J . Amer. Chem. Soc., 89, 4611 (1967); (b) P. C. Turley and P. Haake, ibid., 4618 (1967); (c) E. W. Abel, R. P. Bush, F. J. Hopton, and C . R. Jenkins, Chem. Commun., 58 (1966). (6) To assess the pyramidal nature of the alkylated sulfur in this system, we have summed the three bond angles about S(1) and compared that sum with tricovalent sulfur in other systems. The larger the sum, the greater the s character in the sulfur u-bonding orbitals and the closer the sulfur is to the 360" value for a planar, trigonal atom. The values for representative systems are as follows: 322" (present C H ZJ.) ~Bennett, F. A. case); 316" in R ~ ~ C I S ( C H S S C H ~ C H ~ S[M. Cotton, and R. A. Walton, Proc. Roy. Soc., Ser. A, 303, 175 (1968)); 315" in Ni(TSP)Cl+ (L. P. Haugen and R. Eisenberg, Inorg. Chem., 8 > 1072 (1969)); 310" in (CH3)2(CeHs)SL(A. Lopez-Castro and M. R. Truter, Acta Crysfallogr.,17, 465 (1964)). (7) (a) W. McFarlane, Chem. Commrm., 700 (1969); (b) J. Chatt, G. J. Leigh, A. P. Storace, D. A. Squire, and B. J. Starkey, J . Chem. SOC. A, 899 (1971). (8) G. N. Schrauzer and H. N. Rabinowitz, J . Amer. Chem. SOC., 90, 4297 (1968); G. N. Schrauzer, Accounts Chem. Res., 2, 72 (1969). (9) Alfred P. Sloan Foundation Fellow, 1972-1974.
Journal of the American Chemical Society 1 96:15
July 24, 1974
and Mr. Robert Curran for his help with early parts of this study. Supplementary Material Available. The table of final positional and thermal parameters for the structure will appear following these pages in the microfilm edition of this volume of the journal. Photocopies of the supplementary material from this paper only or microfiche (105 X 148 mm, 24X reduction, negatives) containing all of the supplementary material for the papers in this issue may be obtained from the Journals Department, American Chemical Society, 1155 16th St., N.W., Washington, D. C. 20036. Remit check or money order for $3.00 for photocopy or $2.00 for microfiche, referring to code number JACS-74-4994. Donald G . VanDerveer, Richard Eisenberg* 9 Department of Cliemistry, Utiicersity of Rochester Rochester, New York 14627 Receiced February 26, 1974
A One-Step Synthesis of Benzocyclobutenes Involving Cooligomerization of Linear Mono- and Diacetylenes Catalyzed by ~5-CyclopentadienylcobaltDicarbonyl Sir: We wish to report that commercially available 9 5 cyclopentadienylcobalt dicarbonyl (+-C6H6Co(C0)2) (1) reacts catalytically with linear 1, m-diacetylenes to produce, in good to moderate yield, trimers the formation of which involves the interaction of six acetylene functions. The product-forming intermediates in these reactions can be intercepted either by other acetylenes, leading to a versatile one-step synthesis of benzocyclobutenes, or by nitriles, leading to complex pyridines. Our first observation was made in the reaction of 1,5hexadiyne (2, n = 2) with 1 in refluxing n-octane (con-
@+
